A PDLC material typically consists of a solid, optically clearmatrix, with embedded droplets of a birefringent liquid crystal. See for example U.S. Pat. No. 4,994,204 of Doane et al. granted Feb. 19, 1991 and assigned to Kent State University. A PDLC device is usually constructed using two transparent glass or plastic plates which are spaced closely parallel and sandwich a layer of the PDLC material. The transparent plates are typically provided with a clear, electrically conductive coating of Indium Tin Oxide (ITO) on their opposing, inwardly facing surfaces. The PDLC material between the transparent plates contacts the ITO coating. Typically the layer of PDLC material has a thickness of between ten and fifty microns. The liquid crystal droplets have a size approximately equal to the wavelength in which the PDLC has to operate. Normally, the liquid crystal droplets are randomly oriented and scatter light aimed normal to the plates. When an AC signal of approximately 1 kHz is applied to the ITO coating at, for example, 10 V rms, then the liquid crystal molecules in the PDLC layer rotate and align with the induced electrical field. The bi-refringent liquid crystal and the polymer are pre-selected so that they have an equal index of refraction when the liquid crystal is aligned by an electric field. Thus, when the AC signal is applied the index of refraction is uniform for light arriving normal to the PDLC layer. This allows the PDLC device to be used as a window that can be made opaque by removing the electric field. It is also possible to build flat panel display devices for use with computers using PDLC devices. PDLC devices have also been used as shutters and modulators for visible and infrared optical instruments and related devices such as displays. They can also be used in a reflection mode to provide a display that can be used, for example, with a PC, PDA or cell phone.
For effective use in an optical application the time to clear a PDLC device and the time to close the same are important. For visible light, the time to open is typically 1/1000 second. This time can be significantly shortened using higher strength electrical fields. The closing time for a PDLC device is typically 10 mS and increases at lower operating temperatures. Currently the closing time of PDLC devices is not controlled. It would therefore be desirable to control and accelerate (shorten) the closing time of a PDLC device since this would enhance its potential performance as a light modulator. It would also be desirable to be able to control both the opening and closing times of a PDLC device.
U.S. Pat. No. 5,784,138 of Kollartis et al. granted Jul. 21, 1998 and assigned to Lucent Technologies, Inc. discloses a shutter having a thin film of PDLC composition that is capable of assuming alternative transparent and scattering states for use in a display screen of a teleconferencing system.
U.S. Pat. No. 5,537,242 of Lim grand Jul. 16, 1996 and assigned to Hughes Aircraft Company discloses a millimeter wave phase modulator including a PDLC component.